Friday, November 29, 2013

I've been fascinated by terminal ballistics testing since the early 1990's when I first started reloading. Back then, we received new phone books each year and I was "that guy" who would sweep through the neighborhood asking for the old phone books so they could be "recycled". By recycled, I really meant they would be soaked in my bathtub and subsequently shot at the range. Over several years, I caught many bullets in those hydrated phone books, developed some awesome handloads, and learned that not all bullets were created equal. I really enjoyed the load development and testing, but eventually had to stop due to other life priorities.

Fast forward to 2009. The pocket pistol boom is in full swing and I catch the bug. I start looking for terminal performance data for these short barrel pistols and find very little information is available. I quickly came to the realization that if I wanted terminal performance information, I'd better start tracking down phone books again. Since 2009, I've progressed from phone books, to water jugs, a failed attempt to create an aqueous media bullet trap, SIM-TEST, and finally Clear Ballistics Gel. About two years ago, I started sharing my test results on the blog. Some of the early tests are embarrassing to view now, but I've done my best to constantly improve the quality of the data I capture, and implement the suggestions from readers viewing the tests. One of those suggestions was to cross validate my terminal performance results with the Soft Tissue Ballistic Penetration Model found in Charles Schwartz's book titled Quantitative Ammunition Selection.

When I transitioned from SIM-TEST media to the Clear Ballistics Gel last year, skepticism ran high that this virtually clear, temperature insensitive, reusable gel could really be a suitable substitute for 10% Ordnance Gel. Fans of the Schwartz book would frequently check my test results vs. the Soft Tissue Ballistic Penetration Model and let me know that the results in line with the model. This was a confidence booster for me and I always thought I should locate a copy of the book to see what this model was all about. Last month, I received an iPad for my birthday so I downloaded the Kindle reader app and purchased the Quantitative Ammunition Selection book from Amazon. As I was puzzling through how to integrate the model into my data sheet, I received an unexpected email from Charles Schwartz. (long story how this unexpected contact happened not suitable to cover now) We’ve been working together over the last two weeks and he’s agreed to explain a bit more about the models in his Quantitative Ammunition Selection book and also the specific metrics I will be adding to my data sheet going forward. With that, take it away Charles!

Since I’ve long been a fan of Bruce’s work, especially as it relates to his ammunition tests conducted in the Clear Ballistics Gel test medium, I could not help but feel more than a little honored when he asked me to write a guest article for his blog.

And why not?

I am tremendously impressed by the range of his work and the considerable database of Clear Gel tests that he has amassed. I believe that Bruce’s work stands as strong indication that eventually synthetic test mediums such as Clear Gel will offer the promise of conducting valid, scientifically-repeatable terminal ballistic tests without the attendant annoyance and expense associated with testing in calibrated ten percent ordnance gelatin. Imperfect as it is at this early time, I believe that Clear Gel, and the general class of synthetic soft tissue surrogates known as PAGs (physically associating gels), has great potential in the field of terminal ballistic testing once the material technology matures. Once the material properties of PAGs are improved through R&D to the point that they are able to more accurately represent the formation and cyclic duration of the temporary cavities produced by both high- and low-velocity projectiles, I suspect that PAGs will become more widely accepted and rapidly supplant calibrated ordnance gelatin as the soft tissue simulant of choice due to its superior transparency and insensitivity to ambient temperature.

So, what’s the common interest here?

Well, that’s easy enough to answer. Bruce and I have both gone about pursuing it in different ways, but our ultimate goal is the same. We both want to see the process of terminal ballistic testing made easier and more affordable for the “average guy” without sacrificing validity in the process. Accessibility and accuracy; that’s the goal. Seeing the need for a new process, or a means of updating an old process, is exactly what each of us, in our own way, has been striving to achieve. This, of course, brings me to the discussion of the way that I addressed what I saw as a need for an approachable mathematical model that eliminates the guesswork necessary (until now) when using water as a terminal ballistic test medium.

Water, just like Clear Gel, is an isotropic substance (that is, it is “the same” in all directions) that is insensitive to ambient temperature and has the added benefit of requiring no calibration in order to produce valid test results. So long as the water has not frozen solid or come to a boil, it will produce nearly identical forces to those that arise in calibrated ten percent ordnance gelatin upon bullet impact. Those forces, related to the speed of sound within the test medium and the density of the test medium, are what determines how the bullet and test medium will interact with one another and the more similar they are, the better their respective behavior will correspond to one another. For the sake of comparison, the speed of sound in water at room temperature is approximately 4,910 feet per second, which corresponds closely to that of the speed of sound in calibrated ordnance gelatin at 4,901 feet per second. The densities of both water and calibrated ten percent ordnance gelatin are also very close to one another at 0.999972 gram per cubic centimeter for water at room temperature and 1.040 gram (± 0.200 gram) per cubic centimeter for ordnance gelatin at 39.2° Fahrenheit.

While water produces expansion rates and bullet weight retention nearly identical to that of calibrated ten percent ordnance gelatin, the one drawback with testing in water is that it does not allow for the proper representation of a bullet’s maximum penetration depth as it occurs in other soft tissue simulants without the use of a penetration depth “conversion factor”. Many of these “conversion factors” exist. Ranging in magnitude from 1.8 to 3.0, these factors see common use, and occasionally, bitter debate, in the self-defense and firearms enthusiast communities. However, the problem inherent with the use of these factors is that no correlative study exists that confirms the validity of any of these values.

Of course, such modeling has been attempted before and more than once, but the problems with these prior attempts are manifold. Some attempts at modeling terminal ballistic performance are simply too subjective or are poorly defined. That is, they are composed of otherwise meaningful physical variables that have been cobbled together, often haphazardly, in the hope that some significant perspective or interpretation might be gained from them. Other models, while capable of producing highly correlated, objective, dimensional results, are simply too esoteric for the average firearms enthusiast’s or concealed weapons carrier’s use because they either rely upon advanced mathematics far beyond the reach of the “average guy” or because they are presented in such a manner as to make them unreachable for all but a few.

Fortunately, it doesn't have to be this way.

The solution to this issue is to produce a mathematical model of high correlation that accurately predicts penetration in soft tissue simulants like calibrated ten percent ordnance gelatin using water as the test medium while still remaining accessible to the "average guy". Of course, that is easier said than done, but that is where Quantitative Ammunition Selection comes to the fore. The mathematical model found in Quantitative Ammunition Selection relies upon a governing expression; a proportionality of high correlation to a population of 735 calibrated ordnance gelatin test data obtained from no less than ten independent, published, and unpublished sources composed of various ammunition manufacturers, laboratories, and law-enforcement agencies. The QAS model, a predictive instrument, which has a correlation of r = 0.942 when compared against those 735 data, possesses a margin of error of ± 1.00 centimeter at a 95% confidence margin when using water as a terminal ballistic test medium. The QAS model’s yields are expressed in units of length (in centimeters or inches) for maximum terminal penetration and mass (in grams or ounces) for permanent wound mass equivalent as these effects would occur calibrated ten percent ordnance gelatin where ordnance gelatin (or soft tissue) is penetrated and permanently crushed through direct contact with the bullet. Using the QAS model, it then becomes possible to make an “apples to apples” comparison of maximum penetration depth and permanent wound mass in tangible units of length and mass between different calibers, bullet weights, and designs; it’s something that only one other model offers, but that one in particular is hardly accessible to the “average guy”.

Of course, there is a procedure for conducting terminal ballistic tests in water. Many people make the unknowing, but completely understandable, mistake of using plastic jugs and containers of all sorts without considering the confounding factors that they are introducing into the process. After all, how else does one confine water so that a bullet can fired into it?

The problem with such a practice is that plastic jugs and containers come in all shapes and sizes and, more to the point, are made of all sorts of different materials having different thicknesses and physical properties. Plastic jugs, often used in such testing, are made from various polymers such as polypropylene (PP), low-density and high-density polyethylene (LDPE and HDPE), polyethylene terephthalate (PET), and polyvinyl chloride (PVC) to name just a few of the materials in common use today. Moreover, plastic jugs often have variable wall thicknesses in their design that vary significantly in strength depending upon their construction and the material from which they are made. These material properties can, and do, influence the expansion rate and weight retention of bullets passing through them, which in turn, can lead to an inaccurate and misleading representation of a bullet’s terminal ballistic performance.

Instead, the best approach is to use a container with the least dimensional variance in its construction and the lowest material strength available. That ‘ideal container’ is the cheapest generic re-sealable one-gallon freezer storage bag that you can find, with “cheaper” being “better” as a general rule.

So, why is “cheaper” better?

Well, because any manufacturer seeks to reduce production costs first through the reduction of material used to manufacture a product, the cheapest re-sealable one-gallon freezer storage bags tend to offer the thinnest, most consistent, container wall possible made of the relatively weak polymer, low-density polyethylene (LDPE). It is a material, so fragile and so thin that you can, with very little effort, push your finger right through it. Try that with a two-liter soda bottle made of PP or PET or a one-gallon milk jug made of HDPE and the effects of these different materials becomes quite apparent. Because re-sealable one-gallon freezer bags are the same thickness throughout their construction (except at the point of closure which is no big deal), they also offer the most consistent test results regardless of where bullet impact occurs, another point solidly in their favor.

Once a bullet being tested has been fired and recovered from the water-filled one-gallon freezer bags, it is necessary to record the data obtained for use in the QAS mathematical model. Velocity at impact, average expanded diameter, and retained weight are entered into the QAS model and the maximum penetration depth in calibrated ten percent ordnance gelatin is predicted. After that, the predicted maximum penetration depth is then used to determine the mass of the permanently damaged soft tissue present within the permanent wound cavity and the exit velocity of the projectile from any thickness of test medium, if that is desired. Because Clear Gel possesses physical properties that are extremely close to those of calibrated ordnance gelatin, the QAS model can also be reasonably assumed to predict terminal ballistic behavior in Clear Gel ballistic test medium.

To illustrate this point, one needs to look only as far as Bruce’s most recent test of the Federal 9mm ‘Tactical Bonded’ 124 gr. JHP (LE9T1) in the Clear Gel test medium.

In the first part of that test, a single Federal 9mm 124 gr. ‘Tactical Bonded’ JHP was fired into a bare block of Clear Gel ballistic test medium. The 9mm JHP struck the test block at a velocity of 1,110 feet per second, expanded to an average diameter of 0.5955 inch, retained over 97% of its initial weight, and penetrated to a depth of 11.625 inches before coming to rest in the Clear Gel test block. Comparing the data produced by this test to the predictions made by the QAS model indicates that the QAS model’s prediction is in strong agreement with this particular test result, predicting a maximum penetration depth for this test of 11.478 inches in the Clear Gel test medium.

In the second part of that test, two shots fired through the IWBA four-layer heavy denim barrier, exited the end of the 16-inch long Clear Gel test block so a concrete value doesn’t exit for those tests. Fortunately, it is possible to predict their respective penetration depths and permanent wound cavity masses using the QAS model, thereby “saving” the tests. In this case, the first test bullet struck the test block at a velocity of 1,095 feet per second after passing through the IWBA denim test barrier. It expanded to an average diameter of 0.494 inch, retaining all of its weight before exiting the end of the test block. Entering the available data into the QAS model, the model predicts that the test bullet would have continued for another 1.293 inches before coming to rest at a maximum penetration depth of 17.293 inches. The second test bullet struck the IWBA denim test barrier and entered the test block at a velocity of 1,116 feet per second, expanded to an average diameter of 0.4865 inch, and retained 99.9% of its original weight before leaving the end of the test block. Using the available data, the QAS model predicts that the test bullet would have continued for another 1.981 inches before coming to rest at a depth of 17.981 inches.

The QAS model is also capable of predicting the residual velocity (or exit velocity) of the two test bullets as they left the 16-inch long Clear Gel test block. The QAS model predicts that the first test bullet fired through the IWBA denim barrier would have had an exit velocity of 122.068 feet per second and a residual kinetic energy of 4.125 foot-pounds meaning that it would likely not pose a significant physical threat to a bystander located immediately downrange. The QAS model predicts that the second test bullet fired through the IWBA denim barrier would have had an exit velocity of 152.984 feet per second and a residual kinetic energy of 6.438 foot-pounds meaning that it also would likely not pose a significant physical threat to a bystander immediately downrange. Both test bullets expended approximately 86% – 88% of their available kinetic energy within the target rendering them unlikely to produce a lethal down-range threat and, in the process, demonstrating the advantage of using a JHP (when they expand, that is) for self-defensive use.

As illustrated above, one of the benefits of using the QAS model is that it can be used to confirm a test result or, in the case of a test bullet exiting the test medium unexpectedly, to “save” the test by using the available data (impact velocity, retained mass, average expanded diameter) to predict the terminal performance of the otherwise “compromised” test event. Since I wish to substantiate this opinion further than I have in this article, I am extremely interested in conducting a detailed statistical analysis of Clear Ballistics Gel test data in order to determine just how closely the QAS bullet penetration model correlates to actual terminal ballistic behavior in that test medium. I hope to share the results of that analysis later.

Quantitative Ammunition Selection is available domestically and internationally in hardcover, paperback, and eBook formats and may be purchased at www.quantitativeammunitionselection.com . Just select the appropriate link found on the lower third of the ‘Home’ page for the format that you want.

-Charles Schwartz

Thanks very much Charles for taking the time to explain the background of the new metrics I will be adding on all future test reports. I think it's important that readers fully understand the science behind the new metrics and what they represent. Additionally, I'm looking forward to sharing my past and future test results with you so together we can continue to improve the quality of information generated by my testing, as well as others who wish to undertake their own terminal ballistics testing.

2 comments:

If all of this makes good sense to me, a 71 year old great-grandmother, I think anyone interested in ballistics can understand this interview. I especially was impressed with the fact that standard zip-close plastic bags are the best containers to use in measuring impact and its results. Should I mention that I am Charles Schwartz's mother?